1. Area of the Art
The invention relates generally to an apparatus and a process for growing a silicon epitaxial layer, and specifically to an apparatus and a process for growing a silicon epitaxial layer which has an epitaxial surface with a consistent resistivity profile.
2. Description of the Prior Art
In an epitaxial reactor, a phenomenon known as autodoping occurs on substrate silicon wafers which do not have a backside film barrier. Autodoping occurs when the dopant used in the substrate wafer (such as boron for P-type, or phosphorus or antimony for N-type) outgasses from the backside of the wafer, gets caught in the flow of the gases used in the epitaxial reactor, and is redeposited on the front side of the wafer toward the peripheral edge of the wafer. The result of this deposition of dopant causes the resistivity profile of the epitaxial surface to significantly vary radially from the edge to the center of the wafer. The center of the wafer will theoretically have the desired resistivity, then the resistivity will decease following the wafer radially outwardly from the center when boron is used, and will increase following the wafer radially outwardly from the center when antimony or phosphorus is used.
Since autodoping is a very undesirable phenomenon and the resulting variation in resistivity is unacceptable, present technology calls for deposition of a film barrier of a silicon nitride, silicon carbide, or more commonly silicon dioxide, on the backside of the wafer. The film barrier on the backside of the wafer prevents the dopant used in the substrate from outgassing and becoming airborne in the reactor chamber. Often, however, the backside film barrier has to be removed after epitaxial deposition. The removal of the backside film barrier after epitaxial deposition increases the cost of the wafer because of the added steps of both deposition and removal of the film.
Several apparatuses and methods have been used to control the effects of autodoping without the use of a backside film barrier. For example, U.S. Pat. Nos. 5,421,288 and 5,487,358 disclose an apparatus and a process for growing a silicon epitaxial layer. Both patents discuss controlling the effects of autodoping without the use of a backside film barrier. According to the patents, the thickness and the resistivity profile of the epitaxial layer are controlled through the use of independent mass flow controllers (MFC) on center injectors and peripheral injectors.
Another example of epitaxial reactors is the one manufactured by Applied Materials Technology Corporation (AMT). This type of epitaxial reactor uses a mass flow controller (MFC) to control an auxiliary dopant supply that is deposited only into the injectors that effect epitaxial deposition on the center of the wafer. The auxiliary dopant added is such that it matches the autodoping dopant on the peripheral edges of the wafer, theoretically giving a smooth resistivity profile.
The limitation of the MFC design relates to the sensitivity of the MFC. For example, when the AMT epitaxial reactor is used, the reliability of the auxiliary dopant is dependent upon a consistent and well-controlled gas pressure on both sides of the MFC. During normal operation of the reactor, there are some pressure fluctuations in the gas flow line leading to the injector. This results in varying auxiliary dopant concentrations being emitted, and thus an unstable resistivity profile in the epitaxial layer of the wafer.
Therefore, a need exists to provide an apparatus and a process for growing silicon epitaxial layer having a consistent dopant concentration and resistivity profile.